None.
Bulk acoustic wave (BAW) chemical sensors are used to measure the concentration of constituents or analyte in fluids (gases and liquids). These acoustic wave devices are typically constructed of piezoelectric crystals coated on at least one side with a material that has an affinity for the analyte whose concentration is to be measured. The device is placed in the fluid stream containing the analyte to be measured, and the analyte is adsorbed or absorbed onto the coated surface. The amount of analyte adsorbed or absorbed by the acoustic wave device increases the mass of the device and alters the viscoelastic properties at the surface of the device, thereby damping the acoustic wave properties of the device. As a result, the frequency at which the acoustic wave device will resonant is altered.
When the acoustic wave device is incorporated into an electrical oscillator circuit, the change in resonant frequency of the device changes the operating frequency of the oscillator. The concentration of the analyte can be determined by measuring the change in operating frequency of the oscillator circuit over time.
These chemical sensors are designed to operate in specific ranges of environmental conditions, such as temperature (e.g.,xe2x88x9210xc2x0 C. to 50xc2x0 C.) and humidity (e.g., 0% to 90% relative humidity) and are capable of detecting small concentrations, and small changes of concentrations, of the targeted analyte. However, small changes in analyte concentrations can produce small changes in the resonant frequency of the crystal. Thus, for example, a small concentration of analyte being measured might alter the nominal resonant frequency of a 10 MHz crystal by about 200 Hz. Therefore, the detection circuit must be capable of detecting the resonant frequency of the crystal with high accuracy.
However, the viscoelastic properties of the device can be affected by thermal dynamic conditions to which the device is subjected. More particularly, temperature and humidity can xe2x80x9cagexe2x80x9d the characteristics of the crystal, causing permanent alteration of the viscoelastic properties of the crystal. This alteration of viscoelastic properties affects the dynamic characteristics of the device, and hence the velocity of resonance in the crystal forming the device. Alteration of the resonant properties of the crystal often creates inharmonic mode responses, which generate noise in the operating frequency of the oscillator circuit. Therefore, it is important to eliminate the effects of noise in the detection circuit.
This invention utilizes time domain signal processing to reduce the inharmonic noise which distorts the fundamental frequency of a bulk acoustic wave sensor.
One form of the invention is a process for reducing the inharmonic noise which distorts the fundamental frequency of the sensor. A voltage variable capacitor is placed in series with the sensor to create a voltage-controlled oscillator. The voltage-controlled oscillator is placed in parallel with a resonant oscillator to form a circuit having a resonant frequency. A reverse bias direct current (dc) voltage is applied across the voltage variable capacitor to alter its capacitance thereby warping the resonant frequency away from inharmonic noise frequencies.
Another form of the invention is a sensor circuit for use in measuring the concentration of analytes in a fluid. The circuit includes a bulk acoustic wave sensor. A voltage variable capacitor is connected to the sensor. An input supplies a bias warping dc voltage to the capacitor. A resonant oscillator circuit detects the fundamental frequency of the sensor, and produces a resonant signal frequency. The bias dc voltage applied to the voltage variable capacitor warps the resonant frequency of the circuit away from the inharmonic noise frequencies.
In one form, the sensor and capacitor are connected in series to form a voltage-controlled oscillator which, in turn, is connected in parallel to the resonant oscillator.